Silicon nitride (Si.sub.3 N.sub.4) sintered bodies are superior in various characteristics such as mechanical strength, heat resistance and corrosion resistance and thus can be used as high temperature structural material, e.g., in the production of gas turbine parts.
In sintering of silicon nitride, various sintering aids are commonly added to accelerate the sintering process because the sinterabilities of silicon nitride itself are very poor. For example, when silicon nitride sintered bodies are produced by a reaction sintering method, a small amount of oxide layer on the surface of metallic silicon powder inhibits a nitriding reaction. Hence metals of the iron family have been added as sintering aids to accelerate the nitridation or to allow the nitriding reaction to proceed smoothly.
Iron is effective in removing the oxide layer on the surface of metallic silicon powder. Moreover, it acts as a useful nitriding accelerator; i.e., at temperatures above 1,200.degree. C., it reaches with metallic silicon to form a liquid phase FeSi.sub.2, thereby accelerating the nitriding reaction. However, the silicon nitride formed is mainly the .beta.-phase type which is formed by the action of the liquid phase of FeSi.sub.2. The .alpha.-phase content of the final sintered body is as low as about 50% by weight. As is well known, the strength of the silicon nitride sintered body increases with an increase in the .alpha.-phase content. Hence the addition of iron as a nitriding accelerator is not preferred.
Sintering aids added usually remain in the grain boundary phase of sintered bodies. Thus the properties of the oxide layer resulting from oxidation of the sintered bodies greatly vary with the type of sintering aid.
Several reports have been presented on the oxidation mechanism of silicon nitride sintered bodies containing sintering aids other than metals of the iron family. Typical examples of such sintering aids are explained below.
When yttrium oxide is used as a sintering aid, above critical temperature (about 1,200.degree. C.) of the reaction, the diffusion of oxygen passing through an oxide layer is a rate-determining stage, and as the composition of the oxide layer comes close to pure silicon dioxide, the oxidation rate constant becomes small. Although the surface of oxide layer is smooth and free of pores, and the oxide layer is in intimate contact with a base material, cooling results in the formation of cracks between the oxide layer and the base material due to the difference in coefficient of thermal expansion therebetween.
When magnesium oxide is used as a sintering aid, the diffusion of magnesium ions from a base material to an oxide layer is a rate-determining stage. In this case, the rapid diffusion of oxygen through the oxide layer and the pores formed by release of the oxidation product, nitrogen gas, from the sample leads to more increase the weight gain after oxidation and more degradation in strength after oxidation as compared with the case in which yttrium oxide is used as a sintering aid.
The nitriding-accelerating mechanism or oxidation mechanism, as described above, varies with the type of the sintering aid used. It is also greatly influenced by the amount of the sintering aid. Hence, various attempts have been made to remove an oxide layer formed by oxidation and to improve the characteristics of the oxide layer by changing the type and amount of the sintering aid. Nevertheless, sintered bodies having satisfactory performance have not yet been produced.